The practice of painting images dates back over 30,000 years, as shown by the prehistoric cave paintings discovered in many parts of the world, including Africa. Australia, and Europe (e.g., the Chauvet-Pont-d'Arc Cave in the south of France). The Greeks and Romans painted on wood panels—a practice that continued until the use of canvas became widespread during the Renaissance period, particularly during the early 16th century.
Watercolor painting involves the use of semi-transparent pigments rather than the mostly opaque acrylic- and oil-based paints, but is considered to be just as old. However, watercolor painting did not become firmly established as an art medium until the Renaissance. The practice was prominently used for botanical and wildlife illustrations on woodblock, vellum, and paper. Watercolor paintings were eventually used for mapmaking around the time when the practice proliferated during the 18th century, which was particularly the result of the works of several English artists. The most notable watercolor painters of that period are perhaps Paul Sandby, who became known as the “father of English watercolor,” Thomas Girtin, for his landscape paintings, and Joseph William Mallard Turner, who is credited as developing the watercolor painting technique of painting in stages, and of refining an image through a sequence of washes and glazes. Later artists such as John Constable and David Cox are credited as developing the techniques of wiping and scratching out the colors.
Today's artist uses watercolor paints that are a combination of pigments ground into small particles to provide the color, a natural gum arabic or synthetic glycol binder that is used to suspend the pigment, but which permits it to adhere to the paper, various other additives (e.g., a surfactant to reduce surface tension of the liquid to permit it to soak into the paper), and a solvent (i.e., water), which evaporates as the paint dries.
Today's artist also uses paper that is particularly manufactured for such applications, and may be made of wood pulp, at the low end, or from cotton or linen at the high. The fibers of the paper are porous and very absorbent, so the paper is usually lightly treated with sizing (e.g., cellulose), to slow the absorption of the paint, but which may nonetheless permit the pigment granules to penetrate, depending upon the grain size. The ability of certain pigment granules to separate from the binder and settle into valleys within the paper to produce a grainy texture, when dry, is known as granulation. Also, some pigments (e.g., lapis, lazuli, and ultramarine) exhibit certain electrical properties giving them the tendency to be drawn towards each other to form clumps on the paper, to provide a similar grainy texture or speckling known as flocculation.
Watercolor painting is unlike painting with acrylic or oil based paints, which stay as they are laid, as watercolor paints, conversely, are more active, and require knowledge of its behavior. There are two basic techniques for watercolor painting, which are known as wet-on-dry and wet-on-wet painting. With the wet-on-dry technique, the brush applies the watercolor paint onto a dry paper, whereas for the wet-on-wet technique, the paint is applied to a region of paper that had already been saturated with water or watercolor paint. There are a multitude of further delineated techniques, many of which are used only with respect to wet-on-wet applications, each of which produce particular effects. Some of these further techniques are known as back-runs, paint diffusion, pouring color, dropping in color, salt texturing, cling-film, drybrush, edge darkening, etc.
Because of the popularity of the unique effects produced by the various watercolor painting techniques, there has been substantial interest in converting a source image of various different kinds (e.g., an original oil painting, a digital photograph, etc.), into a watercolor painting. This conversion had been done manually by artists, but with the ability to digitize an image and manipulate it using a computer system, methods for digitally converting source images have also arisen. These methods exhibit varying levels of success at realistically mimicking painterly effects to create a realistic simulated watercolor image. The present invention is directed to a process that converts a source image into a simulated watercolor-painted image to more closely resemble the techniques and realism achieved when an artist manually creates the painting using watercolor paints on paper, and which may run efficiently on different devices and architectures, particularly low-power mobile devices.